When Jay-Z celebrates his birthday, he calls DJ Cassidy to provide the soundtrack. So does Naomi Campbell, who has flown him to Saint-Tropez and Dubai for her parties. He has DJ-ed the weddings of J-Lo and Beyonce, and now his bona fides are just about complete: Cassidy is now unofficially being called President Obama's DJ.

Cassidy doesn't think too much in advance about what he's going to play.

"Of course I have an idea of what I'm going to play at the inauguration. But the real skill is being able to not play what you thought you were going to," he says.

Technology has helped with that somewhat. "For years, I carried eight crates of records, and I traveled with those records all over the world," he says. "Now, you get to carry 30,000 songs on your laptop instead of 1,000 songs in eight crates of records."